Arsenic in the coal studied is not bound to coal macerals, it is concentrated in the inorganic part of coal, specifically in the iron sulfides. This paper reports the results of five crystal structure refinements that include natural (2) and synthetic (3) sulfides, pyrite (3) and marcasite (2). Powder (3) and single-crystal (2) intensities used in the refinements were obtained by diffraction of X-rays (4) and synchrotron radiation (1). The refined occupancy factors of sulfur and the unit-cell parameters favor the possibility that As substitutes for S in both disulfides. (c) 2006 Elsevier B.V. All rights reserved.